1. Field of the Invention
The present invention relates to an optical transmission system for transmitting data by multiplexing a plurality of optical signals having different wavelengths. More specifically, the present invention relates to an optical add-drop multiplexer for dropping or adding an optical signal having a specific wavelength out of or to multiplexed optical signals.
2. Description of Related Art
Wavelength-division-multiplexed optical transmission systems for multiplexing optical signals having different wavelengths and transmitting the multiplexed signals through an optical fiber use an optical add-drop multiplexer. The optical add-drop multiplexer drops an optical signal having a specific wavelength to be received in a communication node, or adds an optical signal to be transmitted in this communication node to the above-described wavelength-division-multiplexed optical signals.
FIG. 2 is a construction diagram of a communication node 12 using optical add-drop multiplexers 3-A and 3-B. The communication node 12 is connected to adjacent communication nodes through a plurality of optical fibers 1, 5, 6 and 11. Wavelength-division-multiplexed optical signals are transmitted between communication nodes through different optical fibers in respective specified directions. In FIG. 2, for example, the optical fibers 1 and 5 are used for a signal to be transmitted from left to right on the paper, whereas the optical fibers 11 and 6 are used for a signal to be transmitted from right to left on the paper.
The communication node 12 needs the optical add-drop multiplexers for dropping an optical signal having a specific wavelength out of wavelength-division-multiplexed optical signals transmitted from an adjacent communication node, and for adding an optical signal to be transmitted from the communication node 12 to the wavelength-division-multiplexed optical signals. That is, the communication node 12 needs one optical add-drop multiplexer for each direction where a signal is transmitted (in FIGS. 2, 3-A and 3-B).
This communication node construction needs an optical add-drop multiplexer for each transmission direction (its example is described in Japanese Patent Laid-open No. 10-20143). Therefore, there arise problems in which the communication cost is increased due to this expensive device, the probability of failure doubles due to the complicated device, as a result, reliability of the communication system falls, and an office space is largely occupied.
A problem to be solved by the invention is that bidirectional transmission between communication nodes needs a plurality of optical add-drop multiplexers, which increases the communication cost.